Spectrophotometric Determination of Tantalum in Iron, Steel, and Niobium Metal YACHIYO KAKITA and HlDEHlRO GOT0
The Research Institute for Iron, Sfeel and Other Metals, Tohoku University, Sendai, Japan A procedure for the determination of small amounts of tantalum in iron, steel, and niobium metal is described. Tantalum forms a complex salt with malachite green in the presence of hydrofluoric acid, and this salt i s extracted with benzene or xylene. Usually boron i s the only element present in the sample which undergoes a color reaction similar to that of tantalum, and it can be removed easily by fuming with sulfuric acid to volatilize it as boron trifluoride. After dissolution of the sample with sulfuric acid, the solution is diluted with ammonium oxalate solution. Hydrofluoric acid is added to a part of this solution, the sulfuric acid concentration i s adjusted to about 0.1N with sodium hydroxide, and malachite green i s added. The complex salt is extracted with benzene, and tantalum i s determined by measurement of the absorbance of the benzene layer at 635 mfi, From 0.005 to 0.5% of tantalum can be determined with good accuracy without any preliminary separation of tantalum. This method should also be applicable to a somewhat wider range (from 0.001 to 5%) of tantalum.
S
as stainless steel are used to a large evtent as a reactor material, but the prescnce of tantalum, r3-hich has a large cross section for absorption of thermal neutrons, results in the formation of a long-life radioactive isotope. Therefore, it is desirable to reduce the tantalum content as much as possible. ,4t present, it is believed that the content of tantalum should be less than 0.017c. For this reason, an accurate and simple method for the determination of small amounts of tantalum is required. The existing method for determination of micro amounts of tantalum in iron and steel requires precipitation and separation of tantalum, together with niobium, by hydrolysis, follom-edby a photometric determination using pyrogallol ( 1 , 2, 6 ) . Pyrogallol should not be used in the presence of titanium, tungsten, and vanadium, and separation by hydrolysis results in incomplete separation of titanium, which affects determination values to a great extent. For the complete separation of titanium, a method PECIAL STEELS such
618
ANALYTICAL CHEMISTRY
final sulfuric acid concentration 0.1N, using phenylarsonic acid has been refollowed by 1.5 ml. of 0.2% aqueous ported (3). Another spectrophotometmalachite green solution. The whole ric determination of tantalum includes volume was then brought to 10 ml. measurement of the ultraviolet specExactly 5 ml. of the organic solvent was trum of tantalum as its peroxide (4, added to this mixture, and the whole but this method also requires a prelimwas shaken vigorously for 30 seconds. inary separation. Tantalum forms a The aqueous layer was drained, and the complex salt with methyl violet in the absorbance of the benzene layer was presence of hydrofluoric acid, and this measured. salt is extracted n-ith benzene. The Selection of Organic Solvent. T h e methyl violet method was applied to complex salt of tantalum a n d malathe determination of tantalum in ores chite green formed in the presence (6). I n this work, malachite green, of of hydrofluoric acid was extracted the same triphenylmethane series as with several organic solvents. T o 5 pg. methyl violet, was used for the deterof tantalum were added 0.1N sulfuric mination of tantalum, and suitable deacid, 2 ml. of 1.5N hydrofluoric acid, termination conditions were examined. and 1.5 ml. of malachite green solution, Malachite green is easier to use than and the volume was brought to 10 ml. methyl violet and has a somewhat larger This \\-as extracted m-ith 5 ml. of an molar absorptivity. Results were satorganic solvent, and the results are isfactory for the determination of tangiven in Tables I and 11. talum in iron, steel, and niobium metal. As indicated in Table I, the complex salt is extracted almost completely with SPECTROPHOTOMETRIC DETERMINATION OF benzene or xylene without another exTANTALUM traction. The salt is not extracted a t Reagents. STAXDARDTANTALUM all with carbon tetrachloride, while the SOLUTIOX.I n a platinum dish, place dye itself is extracted with ethyl acetate, 250 mg. of tantalum metal (purity, amyl alcohol, chloroform, methyl iso99.9%). About 10 ml. of 47% hybutyl ketone, and diethyl ether. These drofluoric acid was added plus nitric gave large blank values, and conseacid, and t h e mixture was heated slightly to effect dissolution. Then, quently, the absorbance could not be 5 ml. of sulfuric acid (18N) was measured. Also, as indicated in Table added, and the solution was heated until uhite fumes evolved. When cooled, sulfuric acid (18N) was added, and the solution was transferred to a 500-ml. measuring flask and diluted to Table 1. Effect of Extraction Solvent the mark M ith 1 8 s sulfuric acid. This Molar is used L I S the stock solution. To 4 ml. ;ibAbof this stock solution, 8 nil. of sulfuric Organir sorb- sorpacid (18s) TKS added, and the whole aiicea tivity Rernnrlis Solvent was diluted to 100 nil. with 4% amBenzene 0 425 76,000 monium oxalate solution. This solu0.162 29,000 Xylene tion contains 20 Wg. per ml. of tantalum. -4myl 0.045 SULFURTC A % ~ ~ ID ~ SOLU- O ~ ~acetat? ~ ~ ~ Carbon tet- 0 00 Kot TION. Twelve milliliters of sulfuric rarhloextracted acid (1SX) was diluted to 100 ml. Trith ride 47c ammonium ovalate solution. Ethyl ,411 the reagents used in this experiacet at c D ment tTere either reagent grade or Amyl certified grade. alcohol Apparatus. Hitachi photoelectric Chloroforni spectrophotometer T y. p. e EPU-2A, 1Ilethyl c i i . glass cell. isobutyl ketone Procedure. Water and 2 ml. of Diethyl 1.5K hydrofluoric acid were placed in b ether a 100-ni1. polyethylene separatory funnel, and a definite quantity of Measured at 635 n1p. * Blank values too large to permit standard tantalum solution was added. measurement. Sulfuric acid (1N) or sodium hydroxide solution ( 2 N ) was added to make the 0
Effect of Various Ions. ANALOGOUS Boron gives the same color change as tantalum, b u t its reaction mith hydrofluoric acid is slow, a n d t h e reaction of boron with malachite green is incomplete. Iodine a n d nitrate ions react similarly. Chloride ion alone has no effect, b u t antimony, if present at t h e sarne time, reacts with chloride ion and gives the same color. Effect of Other Substances. T h e addition of around 0.5 gram of sodium sulfate has no effect, b u t a larger amount tends to give l o ~ c r values. T h e effect of sodium chloride is greater, a n d its presence in amounts larger t h a n 0.2 gram gives a lower value. ilddition of sodium perchlorate causes precipitation of t h e dye and interferes in the determination of tantalum. Addition of phosphoric acid (monosodium phosphate and disodium phosphate mere used) increascs the absorbance and interferes in the determination, but the aniount of pht 5phorus present in iron and steel has no effect. Addition of ammonium ovalnte has no effect u p to about 0.1 gram, but in larger amounts, it tends to give lowcr values. Up to 0.1 gram of sodium tartrate and sodium citrate do not affect the reaction. REACTIONS.
w u
as
2 a
0.f
2
E
a3
Q:
0.2
m
0. I 500
550
600 650 WAVE L E N G T H ( m y )
700
750
Figure 1 . Absorbance curves of tantalum-malachite green complex in benzene solution
X 0
0
Blank Tantalum 1 pg. per ml. Tantalum 1 pg. per ml.
11, both xylene and benzene are good rxtractants, but xylene is inferior to benzene as shown by the molar absorptivity. Accordingly, benzene with a higher sensitivity was used for subsequent experiments. Absorbance Curves. Absorbance curves were obtained with benzene as t h e extraction solvent using t h e conditions given above. T h e absorbance of t h e benzene solution was measured in the range of 450-750 mp, with benzene as the reference. absorbance curves thereby obtained are shown in Figure 1 whcre absorhances beloa500 nip are omittcd. As Figure 1 shows, absorbance is maximum at around 635 nip, as is also the case when sylrne is used. Extraction a n d Acidity of Solution. Extraction is effected in a weakly acid solution. T h e folloa-ing experiments were carried out to determine the optinial acidity. T o 6 pg. of tantalum containing 0.5 nil. of 2 5 sulfuric acid were adtied 2 nil. of 1.5'Y liyrlrofluoric acid solution and 1.5 ml. of malachite green solution, and the concentration of froe sulfuric acid of thi? solution \vas adjusted with sodium hydroxide or sulfuric acid. The solution was dilutcd to 10 nil., and thc p H was measured w i t h T6y6 p H tcst papr'r. 'The cotnples !vas extractrd with 5 nil. of benzenr, and the nbsorlxince of the extract was inensured :it 635 i n p . The results are g i v m hi l'nl de 111. As iiidicatcd in T a b h 111. absorbance is the greatcst and the blank value comparatively small when the complex salt is cxtractcd a t a sulfuric acid concentration of about 0.1S and in a pH range of 0.4 to 0.7. Amount of Malachite Green Solution Added. T h e amounts of malachite green solution added were varied keeping the other factors t'he same. Results are given in Table IV. ;is shown in the table, a constant value of absorbance is obtained by addition of over 1.25 nil. of nialchite green solution. If t h e amount of dye solu-
+ blank
tion added is too large, the blank value tends to increase. Amount of Hydrofluoric Acid Added. In other experinleiits, only t h e amount of 1.5N hydrofluoric acid solution added R as v a r i d , and results are listed in Table T'. T h e results indicate t h a t the addition of 1.5 to 3.0 ml. of hydrofluoric acid solution is most satisfactory. Relationship between Amount of Tantalum a n d Absorbance. Various amounts of tantalum n e r e used in other experiments using the procedure as described. These results listed in Table V I show t h a t t h e relationship betv een the aniount of tantalum and absorbance is linear. T h e molar absorptivity a t this wavelength is 76,000.
Table II.
Organic Solvent Benzeiie
DETERMINATION OF TANTALUM IN I R O N , STEEL, A N D N I O B I U M METAL
Of various elements present in iron and steel and especially in stainless steel, only boron interferes in thc dcterniiiintion of tantalum. Hone\ cr. its
Effect of Number of Extractions
Absorbancea -_______ First Extraction Second Extraction Measured Average 1Ieasured .4verage 425 0 421 0 425 0 00-3 0 001 0 00'' 426 0 423 0 002 0 002 162 0 163 0 162 0 001 0 002 0 001 162 0 160 0 001 0 001
0 0 Xylene 0 0 lleasured :Lt 635 mp.
Table 111.
Effect of Acidity on Extraction
Concn. of PH 1 8 1 6 1 2
1 0
a
Free IIZSO,
s
0 025 0 8 0 050 0 7 0 075 0.6 0.10 0.4 0.15 Below 0.20 0.4 0.30 0.40 0.50 Measured at 635 mp.
Absorbance:_____ Color of Aqueous Solution Blue Blue Greenish blue Dark green Dark green Dark green Dark green Dark green Dark brownish green Brownish green Greenish yellow Greenish yellow
__
Blank value
Corrected with blank value
0 103
0 %SO n. . :MI
0 1.V
0.i 3
0.452
0 . 123 0.102 0.072 0,060 0 . os5
0.477 O..J!IX 0.520 0.325
0.045
0.515 0.510
0,030 0.025 0.025
0.520
0.495 0.465
VOL. 34, NO. 6, M A Y 1962
619
Table IV. Effect of Amount of Malachite Green
Amount of 0.270 Malachite Absorbance" Green Corrected Soln. Blank with Added, M1. value blank value 0.031 0.419 0.50 0.038 0.463 0.75 0.048 0.495 1.00 1.25 0.052 0.525 0.059 0.526 1.50 0.079 0.526 2.00 0.105 0.525 3.00 a Measured at 635 mu. Table V.
Effect of Hydrofluoric Acid
Absorbance" Corrected Blank with value blank value 0.035 0.3i5 0.040 0,465 0.047 0.519 0.051 0.525 0.055 0.525 2.00 3.00 0.060 0 520 5.00 0,080 0.500 7.00 0.095 0,485 Measured at 635 mp.
Amount of 1.5N HF Added, M1. 0.50 1.00 1.50 1.75
Table Vi. Relation between Amount of Tantalum and Absorbance"
Ta Taken, pg.
Absorbance Corrected with Blank Value
mixture was heated t o accelerate dissolution. This solution was oxidized by the addition of about 2 nil. of 30% hydrogen peroxide in small portions. The mixture mas heated to accelerate decomposition, 1 ml. of hydrogen peroxide was added, and the mixture was concentrated by heating until salts began to separate out. The salts mere dissolved by the addition of 4% ammonium oxalate solution, and the solution was transferred to a 50-ml. measuring flask and diluted to 50 ml. with ammonium oxalate solution. A 0.5-2.5-ml. portion of this solution was placed in a polyethylene separatory funnel. The subsequent procedure was the same as that used in the preparation of the calibration curve, and the amount of tantalum was calculated from the calibration curve. Samples insoluble in sulfuric acid were dissolved in aqua regia or hydrochloric acid and heated with 6 ml. of 18N sulfuric acid until white fumes evolved to remove nitric and hydrochloric acids completely. This residue was dissolved in ammonium oxalate solution, and tantalum was determined by the same procedure as above. This requires a somewhat
Table
0.340
0.422 0.510 0.598 0.670 Measured at 635 mp; blank value 0.060.
removal by volatilization makes it possible to determine tantalum without a preliminary separation. The following analytical procedure was therefore devised.
Preparation of Calibration Curve. Various amounts of standard tantalum solution containing up to 10 pg. of tantalum were used following the described procedure. The absorbance of the benzene layer was measured at 635 mp, with the blank solution as the reference. Relationship between absorbance and amount of tantalum was plotted to obtain a calibration curve. ANALYTICAL PROCEDURE
Determination of Tantalum in Steel. SAMPLESKOT COSTAINIKGBORON. To 0.06-0.5 gram of sample, 6 ml. of 18N sulfuric acid and a small amount of water were added, and t h e 620
e
ANALYTICAL CHEMISTRY
Analysis of Samples with Tantalum Added
Amount of Sample Sample Taken, Grams 0.1000 Electrolytic iron 0.1000 0.1000
0.082
0.170 0.251
VII.
Stainless steel
0.5000 0.5000 0.5000
Stainless steel (?Ti, 8; Cr, 18; TI, 1.5)
longer time because the precipitated salts are difficultly soluble. SAMPLE^ CONTAINIKG BOROX. The sample mas similarly dissolved in sulfuric acid and water, oxidized with hydrogen peroxide, and transferred to a platinum dish. The solution was concentrated by heating; about 5 nil. of 47% hydrofluoric acid solution was added, and the solution mas again heated until white fumes evolved. After allowing this to cool, the residue was dissolved in ammonium oxalate solution, the volume n a s brought to 50 nil., and tantalum vias determined as in the foregoing procedure. Determination of Tantalum in Niobium Metal or Ferroniobium. A sample was taken in a platinum dish, a few milliliters of 4 i % hydrofluoric acid was added, followed by a fern drops of nitric acid, and the mixture was heated gently to accelerate diasolution. T h e solution n a s heated n i t h 6 ml. of 18.1-sulfuric acid until white fumes evolved. Then. the residue was dissolved in ammonium oxalate solution, the volume was made u p to 50 ml., and tantalum was determined as in the foregoing cases.
0.5000 0.5000
Amount of Ta Added Amount of Ta Found MicroPer MicroPer cent gram grama cent 50 0.05 51 0.051 50 0.050 100 0.10 99 0.099 100 0,100 0.15 150 0.150 150 149 0.149 25 0.005 25 0 005 24 0.005 50 0.010 0,010 50 49 0.010 400 0,080 400 0.080 405 0.081 25 25 0.005 0.005 24 0.005 100
0,020
50
0.050
150
0.150
50
0.50
Electrolytic iron
Stainless steel (B, 0.25: Cr. 23.65: Ni, 20.97; Ti, ' 0.21)
0.1000 0.1000
100
0.100
0.1000
300
0.200
Table VIII.
Composition of Sample, 7 0 S i , 11.00; Cr, 17.49; Mo, 0.22; Ti, trace; Nb, 0.70 Ni, 14.08; Cr, 18.24; Mol 2.62; Ti, 0.10; Nb, 0.65
100
99 51 49 150 149 51
0.020 0.020 0.051 0.050 0.150 0.149 0.051
50
0,050
99 100 200 200
0,099 0,100 0.200 0.200
Analysis of Stainless Steel
mount of Sample, Gram 0,1000
Amount Micrograms 65.0
0.1000
77.5 77.5
66.0
Amount of Ta Found by of Ta Found ~yrogal~o~ Per Method after cent Hydrolysis, % 0.065 0.06 0.066 0,078 0.08 0.078
Table IX.
Sample Niobium metal (Yokosawa Chem.)
Niobium metal (Johnson) Ferroniobium (Nb T a 61%) _.
+
Analysis of Niobium Metal and Ferroniobium
Amount of Sample Taken, Grams 0,0590 0.0500 0.0500 0.0500 0.0500 0.0500 0.0500 0.1000 0.1000 0.0593 0.0.593 0.0226 0.0226
Amount Ta Found of T a Added, fig, Micrograms Per cent 0 138.8 0.235 117.5 0,235 0 0 118.8 0.238 75.0 192.5 ... 75.0 191.3 ... 125.0 241.3 ... 125.0 242.5 ... 0 300.0 0.30 0 300.0 0.30 0 3025 5.10 0 3040 5.15 0 1160 5.13 0 1150 5.09
Amount Obtained by Subtraction of T a in Sample, fig.
... ... 75.0 73.8 123.8 125.0
... ...
as shon-n in the analysis of niobium metal with a known amount of tantalum added, and also that this procedure gives accurate results. As noted earlier, boron is the only element usually present in iron, steel, and niobium metal, n-hich undergoes a similar color reaction. Tantalum and other elements have no effect on this method. Since a simple method for removal of boron has been found, the present procedure is satisfactory, as the analytical results indicate. From 0.005 to 0.5% of tantalum can be determined without any preliminary separation of tantalum, and the author suggests that this method will also be applicable to a somewhat wider range of tantalum (from 0.001 to 54&) than that described above. LITERATURE CITED
RESULTS AND DISCUSSION
A known amount of tantalum was added to steel of various compositions not containing tantalum, and the determination was carried out by the method mentioned above. Satisfactory results were obtained as listed in Table VII. As indicated by the results in Table VII, a satisfactory determination can also be carried out b y heating the sample containing boron with an excess of hydrofluoric acid to form boron trifluoride, and further by heating with sulfuric acid until white fumes evolve to evaporate boron.
Tantalum was determined in stainless steels containing a known amount of tantalum, and the result of the present analytical procedure was approximately the same as that of spectrophotometric determination using pyrogallol, as indicated in Table VIII. Results of the determination of tantalum in niobium metal, ferroniobium, and niobium metal with a h o l m amount of tantalum added are given in Table TX. It is clear from the results in Table IX that the presence of a comparatively large quantity of niobium does not affect the determination of tantalum
(1) Ikenbery, L., Martin, J. L., Boyer, R. J., AXAL.CHEW25, 1340 (1953). (2) Ishibe, I., Hosoda, K., Higashide, H., Rept. 4846, 19th Committee of Japan Society for Promotion of Science, 1957. (3) Kindman, L., Darn-vert, C. L., White, G., Metallurgia 62, 125 (1960). (4) Palilla, F. C., hdler, N., Hiskey, C. P., ANAL.CHEM.25, 926 (1953). (5) Poluetkov, N . S., Kononenko, L. Z., Lauer, R. S.,J . Anal. Chem. U.S.S.R. 13, 449 (1958). (6) Yana, N., Mochizuki, H., Kajiyama, R., Misaki, T., Rept. 3274, 19th Committee of Japan Society for Promotion of Science, 1954. RECEIVEDfor review July 11, 1961. Accepted February 1, 1962.
Analysis of High-Purity Chromium R.
E. HEFFELFINGER,
E. R. BLOSSER, 0. E. PERKINS, and W. M. HENRY
Analytical Spectroscopy Division, Battelle Memorial Institute, Columbus, Ohio
b A combination chemical-spectrographic method for determining metallic impurities in high-purity chromium i s described. Chromium i s removed from the metallic impurities b y volatilization as chromyl chloride. The remaining solution i s examined spectrographically for elements such as iron, nickel, aluminum, manganese, titanium, vanadium, magnesium, and copper in the 0.1to 30-p.p.m. range. Direct arcing of chromic sulfate i s used for the determination of silicon.
C
a metal used for modern high-temperature applications, has good oxidation resistance. cold workability, high-temperature toughness, and availability. Brittleness, one of its less desirable properties, appears to be related to the impurities present. HROhIIUM,
K h e n the importance of the purity of chromium was first learned, the only satisfactory method for the determination of metallic impurities was the spectrographic analysis by d.c. arc excitation of chromic sulfate which permitted detections in the order of 10 to 50 p.p.ni. Hon-ever, it soon heedme necessary to improve both the detectability and the accuracy of the determination of the impurity elements. To accomplish this, we selected a chemical separation step-volatilization of chromium as the chromyl chlorideprior to spectrographic analysis. The impurities were thus concentrated from a large amount of sample into a small volume of solution free of the matrix material. Then the highly reproducible spark excitation could be used with solution standards to determine these concentrated impurities. Others have
added the separated impurity elements to a powder matrix such as calcium carbonate to handle the very small amount of impurity and then completed the analysis by d.c. arc spectrographic technique ( 2 ) . The solution spark offers greater precision and accuracy because of the inherently higher Irecision of spark and>.sis and because of less handling of thc impurities PROCEDURE
Dissolve 5.lb5.5 grams of chromium in 80 ml. of &I7 hydrochloric acid and dilute in a 100-nil. graduated cylinder to give a chromium concentration of 0.1 gram per ml. The excess above 50 ml.-i.e., 1-5 ml. -may then be tapped off and put into a crucible with 2 nil. of concentrated sulfuric acid, dried, and ignited a t 800°C. to obtain chromic sulfate which is analyzed by ordinary d.c. arc techniques to deVOL. 34, NO. 6, M A Y 1962
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